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Examining the IR pattern for C=O vibrations

November 24, 2008

Infrared (IR) spectrometry is an excellent diagnostic tool for identifying, or lack of, a carbonyl functional group(s). Carbonyl stretching vibration absorbs between 1900-1600 cm-1—a region where few other functional groups absorb. In addition, the carbonyl vibration is typically intense and thus easy to spot.

The gas phase FT-IR spectra, shown below, represent five carbonyl bands differing in the neighbouring atoms: acid halide, carboxylic acid, ester, ketone and amide. The general pattern can be used to suggest a carbonyl group based on the wavenumber.

IRforC=O_Nov

TIP: conjugated carbonyl groups and hydrogen bonding will typically shift the C=O band to a lower frequency. Ring strain and heteroatoms will shift the C=O band to a higher frequency.

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